Gas flotation tanks are used to separate unwanted phases or contaminants such as hydrocarbons from produced water generally by allowing or facilitating the rising of the unwanted phases or contaminants to the surface of produced water. The hydrocarbons may then be removed via skimming of the surface of the produced water.
One typical gas flotation tank comprises of a number of chambers separated by a dividing wall but in fluid communication with each other. During operation, produced water is input into the tank and a rotational current is generated encouraging hydrocarbon to rise to the surface of the water while forcing cleaner more purified water towards the bottom of the tank. By passing the lower water to an adjacent chamber, each successive chamber contains produced water having a lower content of hydrocarbons until a desired purity level is reached and the water is output from the gas flotation tank. One problem with such a design is the need for heavily reinforced divider walls between each chamber of the gas flotation tank as fluid levels in each chamber can be unequal and the difference in fluid level can be significant enough to damage the divider wall and the tank. In addition, depending on the location of the fluid communication port between each divider wall of the tank, water can short circuit across the chambers resulting in water in the final chamber being output with a higher than desirable hydrocarbon content.
To avoid short circuiting, one gas flotation tank includes an interconnecting pipe to connect the chambers in series without creating a short circuit from the inlet to the outlet. The interconnecting pipe is located in such a way that the water considered to be cleanest is taken from one chamber to the next, released near the surface, and dispersed in a fashion (in conjunction with a water weir) to create a flow pattern and velocities that facilitate skimming of the surface hydrocarbon towards an oil skimming trough. The interconnecting pipe also acts as a region in which “micro-bubbles” may be introduced before entering subsequent chambers to ensure even mixing with flow going into each chamber.
However, the interconnecting pipe allows for, in an upset condition, for example an uncontrolled increase/decrease inlet flow, a large level difference that can collapse the internal walls hence requiring a need to heavily reinforce the tank. In order to minimize the risk of large level differences the interconnecting pipe size can be increased. However, such an increase can obstruct the flow pattern within the tank as well as reduce the working volume of the chamber thus rendering the tank less efficient. In addition, such an interconnecting pipe is limited by standard pipe and rolled plate sizes and associated costs and furthermore, filling and draining the tank is a delicate process that requires careful monitoring of the level in each chamber.
Another type of flotation tank is referred to as a serpentine tank and includes a number of chambers, each chamber separated by a partition wherein a portion of the partition is a perforated plate or opening, allowing for the balancing of the chambers. However, a serpentine tank allows only for horizontal flow through the tank, wherein gravity and time are used for the separation of the unwanted phases. The fluid in a serpentine tank flows substantially in one direction inside the chamber (lengthwise) and exits the chamber through the perforated plate, or open section, to the adjacent chamber where it flows horizontally the length of that chamber repeating for as many chambers as is provided in a given tank, hence the term “serpentine”. This pattern of going end to end also creates the requirement for individual skimming points in each chamber, which would then also require additional nozzles on the tank, external piping, and valves for removing the unwanted phases.
A need therefore exists for a gas flotation tank that mitigates short circuiting while reducing or removing the dependency on interconnecting piping.